1 Answer
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I believe the problem is that you've created the subdomain "blog.mydomain.com" already, and are trying to recreate that subdomain with the CNAME record. Try deleting the subdomain you've already created, and then try adding the CNAME record.

I wasn't aware the CNAME is INSTEAD of a sub-domain, and that it should be part of the DNS of mydomain.com. Thanks!
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NoamJul 15 '13 at 6:43

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Creating a "subdomain" the way you initially did simply creates an A-record for that hostname. An A-record must have an IP address associated with it, whereas a CNAME record essentially just forwards to the request to the new host defined in the record. Most hosting companies could probably make this clearer to users, as it's not always obvious.
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nathangiesbrechtJul 15 '13 at 15:38